Carbon nanotubes (CNT) are nanometer-scale sized tube-shaped molecules having the structure of a graphite molecule rolled into a rube. A nanotube can be single-walled or multi-walled, dependent upon conditions of preparation. Carbon nanotubes typically are electrically conductive and mechanically strong and stiff along their length. Nanotubes typically also have a relatively high aspect ratio (length/diameter ratio). Due to these properties, the use of CNTs as reinforcements in composite materials for both structural and functional applications would be advantageous.
However, there are several drawbacks associated with carbon nanotube-reinforced composites. First, CNTs are known to be extremely expensive due to the low yield and low production and purification rates commonly associated with the current CNT preparation processes. The high material costs have significantly hindered the widespread application of CNTs. Second, it is well-known in the field of composites that the reinforcement fiber orientation plays an important role in governing the mechanical and other physical properties of a composite material. However, CNTs tend to form a tangled mess resembling a hairball, which is difficult to work with. This and other difficulties have limited efforts toward realizing a composite material containing well-dispersed CNTs with desired orientations.
Instead of trying to develop much lower-cost processes for making CNTs, researchers (Jang, et al.) at Nanotek Instruments, Inc. have worked diligently to develop alternative nano-scaled carbon materials that exhibit comparable properties, but are more readily available and at much lower costs. This development work has led to the discovery of processes for producing individual nano-scaled graphite planes (individual graphene sheets) and stacks of multiple nano-scaled graphene sheets, which are collectively called nano-sized graphene plates (NGPs). NGPs could provide unique opportunities for solid state scientists to study the structures and properties of nano carbon materials. The structures of these materials may be best visualized by making a longitudinal scission on the single-wall or multi-wall of a nano-tube along its tube axis direction and then flattening up the resulting sheet or plate (FIG. 1(a)). FIG. 1(b) shows an atomic force microscopic picture of a sample of NGPs. In practice, NGPs are obtained from a precursor material, such as minute graphite particles, using a low-cost process, but not via flattening of CNTs. These nano materials could potentially become cost-effective substitutes for CNTs or other types of nano-rods for various scientific and engineering applications.
Specifically, Jang, et al. disclosed a process to readily produce NGPs in large quantities [B. Z. Jang, L. X. Yang, S. C. Wong, and Y. J. Bai, “Process for Producing Nano-scaled Graphene Plates,” U.S. patent application Ser. No. 10/858,814 (Jun. 3, 2004)]. The process includes the following procedures: (1) providing a graphite powder containing fine graphite particles preferably with at least one dimension smaller than 200 μm (most preferably smaller than 1 μm; (2) exfoliating the graphite crystallites in these particles in such a manner that at least two graphene planes are fully separated from each other, and (3) mechanical attrition (e.g., ball milling) of the exfoliated particles to become nano-scaled, resulting in the formation of NGPs with platelet thickness smaller than 100 nm. The starting powder type and size, exfoliation conditions (e.g., intercalation chemical type and concentration, temperature cycles, and the mechanical attrition conditions (e.g., ball milling time and intensity)) can be varied to generate, by design, various NGP materials with a wide range of graphene plate thickness, width and length values. We have successfully prepared NGPs with an average length smaller than 500 nm and, in several cases, smaller than 100 nm. Ball milling is known to be an effective process for mass-producing ultra-fine powder particles. The processing ease and the wide property ranges that can be achieved with NGP materials make them promising candidates for many important engineering applications. The electronic, thermal and mechanical properties of NGP materials are expected to be comparable to those of carbon nano-tubes; but NGPs will be available at much lower costs and in larger quantities.
The NGP material can be used as a nano-scaled reinforcement for a matrix material to obtain a nanocomposite. Expected advantages of nano-scaled reinforcements in a matrix material include: (1) when nano-scaled fillers are finely dispersed in a polymer matrix, the tremendously high surface area could contribute to polymer chain confinement effects, possibly leading to a higher glass transition temperature, stiffness and strength; (2) nano-scaled fillers provide an extraordinarily zigzagging, tortuous diffusion path that results in enhanced barrier or resistance against permeation of moisture, oxygen, other gases, and liquid chemical agents. Such a tortuous structure also serves as an effective strain energy dissipation mechanism associated with micro-crack propagation in a brittle matrix such as ceramic, glass, or carbon; (3) nano-scaled fillers can also enhance the electrical and thermal conductivities in a polymer, ceramic or glass matrix; and (4) carbon-based nano-scaled fillers have excellent thermal protection properties and, if incorporated in a matrix material, could potentially eliminate the need for a thermal protective layer, for instance, in rocket motor applications.
In a related subject, exfoliated graphite may be impregnated with a resin to obtain an expanded graphite flake (EGF)-resin composite. Alternatively, expandable graphite particles may be dispersed in a monomer or oligomer and then exfoliated before the monomer/oligomer is polymerized or cured, also resulting in the formation of an expanded graphite flake-resin composite. These conventional exfoliated graphite flake composites are discussed in the following references:    1. P. Xiao, L. Y. Sun, M. Xiao, and K. C. Gong, “Preparation and Properties of Polystyrene/Graphite Nanocomposite,” Materials Res. Soc. Symposium, 661 (2001) KK5.3.1-KK5.3.6.    2. P. Xiao, M. Xiao, and K. C. Gong, “Preparation of Exfoliated Graphite/Polystyrene Composite by Polymerization-filling technique,” Polymer, 42, 4813 (2001).    3. M. Xiao, L. Y. Sun, J. J. Liu, Y. Li, and K. C. Gong, “Synthesis and Properties of Polystyrene/Graphite Nanocomposite,” Polymer, 43-8 (2002) 2245.    4. H. Shioyama, “Polymerization of Isoprene and Styrene in the Interlayer Spacing of Graphite,” Carbon, 35 (1997) 1664.    5. H. Shioyama, “Review: The Interactions of Two Chemical Species in The Interlayer Spacing of Graphite,” Synthetic Metals, 114 (2000) 1.    6. G. H. Chen, D. J. Wu, W. Weng, B. He, W. Yan, “Preparation of Polymer/Graphite Conducting Nanocomposites by Intercalation Polymerization.” J. Appl. Polymer Sci., 82 (2001) 2506-13.    7. G. H. Chen, C. Wu, W. Weng, D. Wu, and W. Yan, “Preparation of Polystyrene/Graphite Nano-sheet Composite,” Polymer, 44 (2003) 1781-1784.    8. G. H. Chen, D. Wu, W. Weng, and C. Wu, “Exfoliation of Graphite Flake and Its Nanocomposites,” Carbon, 41 (2003) 619-621.    9. G. H. Chen, W. Weng, D. Wu, and C. Wu, “PMMA/Graphite Nanosheet Composite and Its Conducting Properties,” Euro. Polymer J., 39 (2003) 2329-2335.    10. G. H. Chen, W. Weng, D. Wu, C. Wu, J. Lu, P. Wang, X. Chen, “Preparation and Characterization of Graphite Nanosheets from Ultrasonic Powdering Technique,” Carbon, 42 (2004) 753-759.    11. J. Zhu, F. M. Uhl, A. B. Morgan, and C. A. Wilkie, “Studies on the Mechanism by Which the Formation of Nanocomposites Enhances Thermal Stability,” Chem. Mater., 13 (2001) 4649.    12. Y. X. Pan, Z. Yu, Y. Ou, and G. H. Hu, “A New Process of Fabricating Electrically Conducting Nylon6/Graphite Nanocomposites via Intercalation Polymerization,” J. Polymer Sci., Part B: Polymer Phy., 38 (2000) 1626.    13. W. Zheng, S. C. Wong, and H. J. Sue, “Transport behavior of PMMA/expanded graphite nanocomposites,” Polymer, 73 (2002) 6767.    14. W. Zheng and S. C. Wong, “Electrical conductivity and dielectric properties of PMMA/expanded graphite composites,” Composite Sci., and Tech., 63 (2003) 225.    15. J. W. Shen, X. M. Chen, and W. Y. Huang, “Structure and Electrical Properties of Grafted Polypropylene/Graphite Nanocomposites Prepared by Solution Intercalation”, J. App. Polymer Sci., 88 (2003) 1864-1869.    16. W. P. Wang and C. Y. Pan, “Synthesis and Characterization of Poly(ethylene Oxide) Methyl Ether Grafted on the Expanded Graphite with Isocyanate Groups,” Euro. Polymer J., 40 (2004) 543-548.    17. H. Fukushima and L. T. Drzal, “Graphite Nanoplatelets As Reinforcements for Polymers: Structural and Electrical Properties,” Proc. Of the 17th Annual Conf. of the Am. Soc. For Composites, Purdue University, (2003).    18. H. Fukushima, S. H. Lee, and L. T. Drzal, “Graphite Platelet/Nylon Nanocomposites,” Proc. of SPE ANTEC (2004) 1441-1445.    19. A. Yasmin, J. J. Luo, and I. M. Daniel, “Processing of Graphite Nanosheet Reinforced Polymer Nanocomposites,” Proc. of the 19th ASC/ASTM Joint Tech Conf., E. Armeiros, Ed., Ga Tech, Atlanta, CDROM, 2004.    20. A. Yasmin and I. M. Daniel, “Mechanical and thermal Properties of Graphite Platelet/Epoxy Composites,” Polymer, 45 (2004) 8211-8219.    21. J. Mack, L. Viculis, A. Ali, R. Luoh, G. Yang, R. Kaner, T. Hahn, and F. Ko, “Graphite Nanoplatelet Based Nanocomposites by the Electrospinning Process,” Proc. Of the 17th Annual Conf. of the Am. Soc. For Composites, Purdue University, (2003).    22. R. S. Caines, “Vermicular Expanded Graphite Composite Material,” U.S. Pat. No. 4,199,628 (Apr. 22, 1980).    23. A. W. Atkinson, D. R. Hurst, and K. T. Somerfield, “Housing for Electrical or Electronic Equipment,” U.S. Pat. No. 4,530,949 (Jul. 23, 1985).    24. D. D. L. Chung, “Low-Density Graphite-Polymer Electrical Conductors,” U.S. Pat. No. 4,704,231 (Nov. 3, 1987).    25. D. D. L. Chung, “Composites of In-Situ Exfoliated Graphite,” U.S. Pat. No. 4,946,892 (Aug. 7, 1990).    26. L. R. Bunnell, Sr., “Enhancement of the Mechanical Properties by Graphite Flake Addition,” U.S. Pat. No. 4,987,175 (Jan. 22, 1991).    27. L. R. Bunnell, Sr., “Enhancement of the Mechanical Properties of Polymers by Graphite Flake Addition and Apparatus for Producing Such Thin Flakes,” U.S. Pat. No. 5,019,446 (Nov. 19, 1991).    28. L. R. Bunnell, Sr., “Method for Producing Thin Graphite Flakes with Large Aspect Ratios,” U.S. Pat. No. 5,186,919 (Feb. 16, 1993).    29. T. P. Hayward, “Method of Making Graphite Foam Materials,” U.S. Pat. No. 5,582,781 (Dec. 10, 1996).    30. L. T. Drzal and H. Fukushima, “Expanded Graphite and Products Produced Therefrom,” U.S. patent application Ser. No. 10/659,577 (Sep. 10, 2003).
These prior-art composites have the following drawbacks:    A). Although the expanded graphite flake (EGF) thickness is typically smaller than 100 nm, the width and length (or diameter) of the plate-like fillers in these composites are typically much greater than 1 μm and more typically in the range of 10-200 μm [e.g., Refs. 6-9 (TEM micrographs), 10 (FIG. 2 & Table 1), 24-30]. Strictly speaking, these fillers are not nano-scaled fillers and the composites are not nanocomposites. In the field of composites, micron-sized fillers often result in the formation of micron-sized defects, hence compromising the composite strength. Only when both the flake thickness is smaller than 100 nm and the flake length is shorter than 1 μm, can the EGFs qualify as NGPs.    B). Due to processing difficulties, those EGF composites with a high EG flake proportion (e.g., >50% by weight) tend to be highly porous and of low strength; e.g., density as low as 0 7 g/cm3 and flexural strength lower than 3 MPa being reported in [Refs. 24 and 25]. The fillers are typically characterized as having substantially un-separated flakes or platelets and having many pores between platelets that are not accessible by the resin. These high-loading EGF composites have been prepared exclusively by “intercalation and in situ polymerization” method (by intercalating graphite particles with a monomer and then polymerizing the monomer in situ, or between graphene layers) [e.g., Refs. 1,2]. For fully separated, individual plates dispersed in a polymer, the viscosity is typically too high to be processable by mass production techniques such as extrusion, injection molding, or even compression molding (hot press molding). In these composites, the weight fractions of graphite flakes are typically smaller than 15% [e.g., Refs. 7-10, 13, 14, 17-19, 30].    C). It has been recently recognized by researchers in the field of composites that thin exfoliated graphite flakes, with extremely high aspect ratio (length/thickness ratio>100˜1000), lead to a lower percolation threshold (typically 1-4% by weight EGF) for forming an electron-conducting path [e.g., Refs. 1, 2, 6-9, 12-15, 17, 18, 30], as compared to a threshold of typically 5-20% for other types of graphite particles. However, at these threshold EGF loadings, the electrical conductivity of the resulting composite, typically in the range of 10−5-10−1 S/cm, is still too low for many useful engineering applications. For instance, the US Department of Energy (DOE) has set forth a target bulk conductivity of 100 S/cm and experts in the fuel cells industry have identified a target areal conductivity of 200 S/cm2 for composite-based fuel cell bipolar plates. Even with high EGF loadings [e.g., Refs. 24,25] in the case of EGF composites, electrical conductivity was typically 2 S/cm or lower.    D). Conventional EGF composites with a high EGF loading either can not be formed into thin composite plate, can not be molded with mass production techniques, or are simply not processable into useful products. Although one would expect the electrical conductivity of an EGF composite to become higher if the EGF loading is greater (e.g., >20% by weight), no melt-blended composite containing more than 20% by weight of well-dispersed, fully separated EG flakes has hitherto been reported. The high EGF loading (up to 62 wt. %) reported by Xiao, et al [Ref. 1] was for highly porous, un-separated expanded graphite sheets clustered together. The composite in this case was formed by “intercalation and in situ polymerization.” Again, these porous structures led to weak composites. Furthermore, the approach of “intercalation and in situ polymerization” is applicable to only a limited number of polymers that have a wide window of synthesis conditions such as polystyrene and nylon-6. This process is typically slow and, hence, expensive. For instance, in situ polymerization of ε-caprolactam to produce nylon-6 requires a reaction time of 20 hours [e.g., Ref. 12]. A vast majority of the existing polymers cannot be prepared in this manner. Alternatively, solution blending was used to prepare EGF/PMMA composites [Refs. 13,14], but residual solvent can be problematic. A need exists for a cost-effective method of preparing directly melt-blended EGF/polymer composites with a high EGF loading.    E). No non-polymeric matrices have hitherto been reported for use in making nanocomposites containing separated EG flakes that are of good structural integrity. Carbon-, ceramic, metal, and glass matrix composites can be used in applications that require high temperature environments.
Thus, it is an object of the present invention to provide a composite that contains fully separated graphite platelets with a sufficient amount effective for achieving a high composite conductivity (greater than 10 S/cm and, preferably greater than 100 S/cm).
It is another object of the present invention to provide a composite that contains fully separated graphite platelets with a sufficient amount effective for achieving a high areal conductivity (greater than 200 S/cm2) for the composite.
It is yet another object of the present invention to provide a composite comprising fully separated graphite platelets that are smaller than 1 μm in length, width or diameter (preferably smaller than 0.5 μm or 500 nm) and smaller than 100 nm in thickness.
Still another object of the present invention is to provide a composite comprising at least 20% by weight of fully separated graphite platelets. A specific object of the present invention is to provide a composite comprising 35%-75% by weight of fully separated graphite platelets. A further specific object of the present invention is to provide a composite, comprising 35%-75% by weight of fully separated graphite platelets, which can be made into a thin plate (thinner than 3 mm, preferably no thicker than 1 mm).
It is still another object of the present invention to provide a composite comprising fully separated graphite platelets that are smaller than 1 μm in length, width or diameter (preferably smaller than 0.5 μm or 500 nm) and smaller than 100 nm in thickness; the composite can be mass produced into desired products (e.g., fuel cell bipolar plates) using cost-effective processing techniques.
It is still another object of the present invention to provide a composite comprising fully separated graphite platelets that are smaller than 1 μm in length, width or diameter (preferably smaller than 0.5 μm or 500 nm) and smaller than 100 nm in thickness; the composite can be mass produced into desired products without involving slow in situ polymerization or use of a chemical solvent.
It is still another object of the present invention to provide a composite comprising fully separated graphite platelets that are smaller than 1 μm in length, width or diameter (preferably smaller than 0.5 μm or 500 nm) and smaller than 100 nm in thickness which are dispersed in a carbon, glass, ceramic, or metal matrix.